An image sensor is a semiconductor device for converting an optical signal to an electrical signal. The image sensor includes a photodiode (PD) for detecting lights and a logic circuit for converting the detected lights to electrical signals.
A conventional image sensor usually has a front side illumination structure, wherein a photodiode is formed below a surface of a substrate, and a logic circuit is formed above the photodiode. Before reaching the photodiode, lights have to pass through the logic circuit and multilayer structures, which results in light loss, or light crosstalk to adjacent image sensor unit chip. Therefore, light response characteristics of each image sensor unit's photodiode are impacted.
In order to solve the above problems, Back Side Illumination (BSI) image sensors have been developed. In the BSI image sensor, lights do not pass through the logic circuit, but directly reach the photodiodes from the back side of the substrate. Thus, the light response characteristics of the photodiode are improved.
However, there is a large dark current in the BSI image sensor. In an operating process of the image sensor, the dark current may be mixed with the signal currents, resulting in interference to the signal and degradation of image sensor performance. More information about the dark current problem of image sensor may refer to a Chinese patent application with a publication No. 103137633A.
Therefore, in order to solve the problem of the large dark current in the conventional BSI image sensor, a new BSI image sensor and a method for reducing dark current of the same are needed.